1. Field of the Invention
This invention relates to an illumination and exposure system for a photographic or reprographic apparatus and more particularly to an improvement in a halftone screen and support for imagewise exposure of a photosensitive member.
2. Brief Description of the Prior Art
An original image that consists of a broad range of tones or gradations of tones is known as a continuous tone image. In several applications of the photographic and reprographic arts, such as in letterpress and offset lithography, such tones cannot be reproduced by varying the amounts of print ink. These printing processes can only print a solid density of color in the image areas, while ink is absent in the non-image areas. In applications of the electrophotographic reproduction art, when reproducing continuous tones, a reproduction can suffer from edge effects and halos. Thus, the reproduced image has a contrasty, washed-out appearance. To provide good image quality, resort may be made to a carefully controlled development process which typically requires the use of special devices, such as a development electrode. Such a technique, and the associated apparatus, can be costly and complex.
In order to reproduce the varying tones of a continuous tone original image, practitioners in graphic arts photography and reprography therefore utilize a half-tone screen. Halftoning, (altering the imagewise exposure by, for example, locating the screen in the optical exposure path) makes the reproduction of a continuous tone image possible by converting the continuous tones into a pattern of very small, clearly defined dots of varying sizes. During exposure of the photosensitive member, the light reflected or transmitted from an original is projected through the transparent portions (or openings) of a screen. Each opening produces a transmission density gradient at the image plane on the photosensitive member. Each dot exposure is proportional in size to the amount of light provided by the respective portion of the original imaged on the opening. Using a stepped gray scale to represent the tones in an original image, a quality reproduction is ultimately provided whereby the darkest area of the original is reproduced as a solid (of large merged dots) and the lightest area is reproduced without a dot (i.e., dots of zero size).
The use of screen patterns and screening processes to improve the rendition of images is known in the fields of photography and electrostatography. See, for example, U.S. Pat. No. 3,493,381, issued in the name of Maurer and U.S. Pat. No. 2,598,732 issued in the name of Walkup. These patents also disclose a variety of screen patterns including lines, circular or rectangular dots, checkerboard patterns, etc.
U.S. Pat. No. 4,227,795, issued to Bobbe et al, is an example of the use of a screening process in multicolor electrography. The disclosed screening process may be carried out by transmitting a light image of the original document through a screen to expose a charged photoconductive member. Alternatively, an image of a screen pattern can be formed on the charged photoconductor either before or after the image of the original document is formed on the photoconductor. In U.S. Pat. Nos. 4,472,047 and 4,537,490, issued to Stoudt, a method and apparatus are disclosed for electrophotographically producing high quality black-and-white and color reproductions of originals having differing types of image content. A half-tone screen is employed in the optical path between the original and a photoconductor image sector.
As mentioned with reference to U.S. Pat. No. 4,227,795, a screen may be used to expose the photosensitive member before or after the member is exposed to the original image. For example, collimated light rays from a small, concentrated point light source are projected to a screen pattern over a rectangular aperture. In such a case, the illumination is directed through the dot screen so as to create a regular pattern of light on the photosensitive member. (A post-imagewise exposure through the screen affords a similar effect.) In either case, a latent image of the original is produced that is substantially equivalent to that provided by the screen when interposed in the imagewise exposure path.
Examples of such pre- or post-exposure screening are disclosed in U.S. Pat. Nos. 4,459,011 and 4,477,177, issued to Day, wherein an apparatus is provided for projecting a dot screen pattern onto a moving photoconductor. The compact screen projector disclosed in U.S. Pat. No. 4,459,011 provides the projection of a screen pattern onto a light sensitive surface, such as a photoconductor. The projecting apparatus comprises a member having a screen pattern including opaque and transparent areas. The member is positioned adjacent the photoconductive surface, and a Fresnel lens is located between the member and the image source. The screen member has thereon a screen pattern comprising opaque and transparent areas. A concentrated source of light rays is spaced from the Fresnel lens, and means are provided for folding light rays from the light source a plurality of times and then directing the light rays onto the lens. In U.S. Pat. No. 4,477,177, the projecting apparatus comprises an elongate transparent cylinder positioned closely adjacent to the photoconductor in the area where the photoconductor travels around a drive roller. The dot pattern is projected onto the photoconductor after it passes the charger and before it reaches an exposure station. However, the cylinder apparatus can be located to project the dot pattern onto the photoconductor after it passes the exposure station.
Quite commonly, the screen in a camera or copier apparatus useable for reproducing a continuous tone or color will be mounted in a horizontal plane. The screen is often mounted in close proximity to the horizontal portion of the photosensitive member. Such a horizontal mounting (of the screen in the optical path of the exposure optics) is also found in a variety of other reprographic and photographic apparatus. It is desirable to interpose the dot screen in the imaging optical path at a small, accurately-controlled distance from the photoconductor image plane. The spacing between screen and photosensitive member is minimal because of the necessary fineness of the dot pattern. To prevent tone scale distortion of the image in an electrophotographic application, for example, the screen must be uniformly separated from the image plane at a distance of typically 0.040 to 0.050 inches (1.0 to 1.3 millimeters).
The screen is typically fixed on the surface of a glass (or similar) substrate which provides mechanical support and optical transparency at the same time. If the image plane is horizontal and the substrate is as broad as the imaging area on the photosensitive member, the dot screen and substrate will usually sag due to the pull of gravity. As illustrated in FIG. 3, the sag at the middle of the image area is typically beyond an acceptable tolerance (for example, in one electrophotographic copier that sag was found to exceed a maximum tolerance of 0.003 inches (0.076 millimeters) from the screen mounting plane). The sag constitutes a variation in the spacing between the screen and the image plane. This variation causes the imaged screen pattern (the imaged dot pattern on the photosensitive member) to be correspondingly distorted. The imaged dot pattern will thus be composed of dots having a non-uniform spacing. The screening process is thus optically impaired and the fidelity of the reproduction to the original is therefore compromised.
The conventional response to this sag induced image distortion is to introduce a compensating distortion (curvature) of the image plane in order to restore the required uniformity of spacing between screen and image plane over the entire image area. However, this approach is difficult, costly, and itself can nonetheless introduce other types of image degradation.
It is an object of this invention therefore to provide for the reproduction of a continuous tone image without the deleterious effects of the conventional screen apparatus described above. A further object is to provide a halftoning screen and support that is not subject to the aforementioned sag and the associated effect of image degradation. It is a still further object to provide a halftoning screen and support that is simple, inexpensive, and easily implemented in any type of reprographic or graphic arts apparatus, and especially in an electrophotographic reproduction apparatus.